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Ab initio quantum mechanical calculations were performed on Al(OH) 3 , Al(OH) 4  - , NaAl(OH) 4  and related species with varying numbers of explicit water molecules to elucidate the structural, spectral and energetic properties of the possible species. We find that Al(OH) 3  reacts with H 2 O in the gas-phase with an exoergicity of 24.1 kcal/mol to produce Al(OH) 3 H 2 O, which has shorter Al-OH distances, larger Al-OH stretching frequencies, and a 15 ppm larger Al NMR shielding than does Al(OH) -  4 . When the first hydration spheres of these species are included the Al NMR shieldings becomes very similar, but the O and H NMR parameters and the IR and Raman spectra still show significant differences. The hydration energy of Al(OH) 3 H 2 O is determined from a “supermolecule"' calculation on Al(OH) 3 H 2 O...6H 2 O, whereas that for Al(OH) -  4  is obtained using the supermolecule calculation on Al(OH) -  4 ...6H 2 O plus an evaluation of the electrostatic Born hydration energy of the supermolecule. The calculated energy change for the acid dissociation reaction, Al(OH) 3 H 2 O...6H 2 O → Al(OH) -  4 ..6H 2 O + H + , is +297.9 kcal/mol in the gas phase but only +2.3 kcal/mol in aqueous solu­tion, due to the strong hydration of H +  and Al(OH) 4  - ..6H 2 O. Using quantum mechanically calculated entropies for the unhydrated species, Al(OH) 3 H 2 O and Al(OH) -  4 , plus the experimental hydration entropy of H + , the -TΔS term for this reaction is calculated as about +11.8 kcal/mol. Adding in calculated zero-point energies and room temperature enthalpy corrections gives a free energy change of +0.5 kcal/mol. Thus pKa for the acid dissociation of Al(OH) 3 H 2 O is near zero at room T, and Al(OH) -  4  will be dominant except under very acidic conditions.

Theoretical studies on aluminate and sodium aluminate species in models for aqueous solution: Al(OH) 3 , Al(OH) - 4 , and NaAl(OH) 4